Mapfile Structure and Syntax

Mapfile directives can span more than one line, and can have any amount of white space, including new lines.

For all syntax discussions, the following notations apply.

• Spaces, or newlines, can appear anywhere except in the middle of a name or value.

• Comments beginning with a hash character (#) and ending at a newline can appear anywhere that a space can appear. Comments are not interpreted by the link-editor, and exist solely for documentation purposes.

• All directives are terminated by a semicolon (;). The final semicolon within a {...} section can be omitted.

• All entries in constant width, all colons (:), semicolons (;), assignment (=, +=, -=), and {...} brackets are typed in literally.

• All entries in italics are substitutable.

• [ ... ]  brackets are used to delineate optional syntax. The brackets are not literal, and do not appear in the actual directives.

• Names are case sensitive strings. Table 8-2 contains a list of names and other strings commonly found in mapfiles. Names can be specified in three different forms.

  • Unquoted

    An unquoted name is a sequence of letters and digits. The first character must be a letter, followed by zero or more letters or digits. The characters percent (%), slash (/), period (.), and underscore (_) count as a letter. The characters dollar ($), and hyphen (-) count as a digit.

  • Single Quotes

    Within single quotes ('), a name can contain any character other than a single quote, or newline. All characters are interpreted as literal characters. This form of quoting is convenient when specifying file paths, or other names that contain normal printable characters that are not allowed in an unquoted name.

  • Double Quotes

    Within double quotes ("), a name can contain any character other than a double quote, or newline. Backslash(\) is an escape character which operates similarly to the way it is used in the C programming language within a string literal. Characters prefixed by a backslash are replaced by the character they represent, as shown in Table 8-1. Any character following a backslash, other than the ones shown in Table 8-1 is an error.

• value represents a numeric value, and can be hexadecimal, decimal, or octal, following the rules used by the C language for integer constants. All values are unsigned integer values, and are 32-bit for 32-bit output objects, and 64-bit for 64-bit output objects.

• segment_flags specify memory access permissions as a space separated list of one or more of the values given in Table 8-3, which correspond to the PF_ values defined in <sys/elf.h>.

Table 8-1 Double Quoted Text Escape Sequences

Table 8-2 Names And Other Widely Used Strings Found In Mapfiles

Table 8-3 Segment Flags

Mapfile Version

The first non-comment, non-empty, line in a mapfile is expected to be a mapfile version declaration. This declaration establishes the version of the mapfile language used by the remainder of the file. The mapfile language documented in this manual is version 2.

        $mapfile_version 2

A mapfile that does not begin with a version declaration is assumed to be written in the original mapfile language defined for System V Release 4 Unix (SVR4) by AT&T. The link-editor retains the ability to process such mapfiles. Their syntax is documented in Appendix B, System V Release 4 (Version 1) Mapfiles.

Conditional Input

Lines within a mapfile can be conditionalized to only apply to a specific ELFCLASS (32 or 64-bit) or machine type.

        $if expr
        ...
        [$elif expr]
        ...
        [$else]
        ...
        $endif

A conditional input expression evaluates to a logical true or false value. Each of the directives ($if, $elif, $else, and $endif) appear alone on a line. The expressions in $if and subsequent $elif lines are evaluated in order until an expression that evaluates to true is found. Text following a line with a false value is discarded. The text following a successful directive line is treated normally. Text here refers to any material, that is not part of the conditional structure. Once a successful $if or $elif has been found, and its text processed, succeeding $elif and $else lines, together with their text, are discarded. If all the expressions are zero, and there is a $else, the text following the $else is treated normally.

The scope of an $if directive cannot extend across multiple mapfiles. An $if directive must be terminated by a matching $endif within the mapfile that uses the $if directive, or the link-editor issues an error.

The link-editor maintains an internal table of names that can be used in the logical expressions evaluated by $if and $elif. At startup, this table is initialized with each of the names in the following table that apply to the output object being created.

Table 8-4 Predefined Conditional Expression Names

The names are case sensitive, and must be used exactly as shown. For example, true is defined, but TRUE is not. Any of these names can be used by themselves as a logical expression. For example.

        $if _ELF64
        ...
        $endif

This example will evaluate to true, and allow the link-editor to process the enclosed text, when the output object is 64-bit. Although numeric values are not allowed in these logical expressions, a special exception is made for the value 1, which evaluates to true, and 0 for false.

Any undefined name evaluates to false. It is common to use the undefined name false to mark lines of input that should be unconditionally skipped.

        $if false
        ...
        $endif

More complex logical expressions can be written, using the operators shown in the following table

Table 8-5 Conditional Expression Operators

Expressions are evaluated from left to right. Sub-expressions are evaluated before enclosing expressions.

For example, the lines in the following construct will be evaluated when building 64-bit objects for x86 platforms.

        $if _ELF64 && _x86
        ...
        $endif

The $add directive can be used to add a new name to the link-editor's table of known names. Using the previous example, it might be convenient to define the name amd64 to stand for 64-bit x86 objects, in order to simplify $if directives.

        $if _ELF64 && _x86
        $add amd64
        $endif

This can be used to simplify the previous example.

        $if amd64
        ...
        $endif

New names can also be added to the link-editor's table of known names by using the link-editor's -z mapfile-add option. This option is useful when mapfile input needs to be conditionally enabled based on an attribute of the external environment, such as the compiler being used.

The $clear directive is the reverse of the $add directive. It is used to remove names from the internal table.

        $clear amd64

The effect of the $add directive persists beyond the end of the mapfile that uses $add, and is visible to any subsequent mapfile that is processed by the link-editor in the same link operation. If this is not desired, use $clear at the end of the mapfile containing the $add to remove the definition.

Finally, the $error directive causes the link-editor to print all remaining text on the line as a fatal error, and halt the link operation. The $error directive can be used to ensure that a programmer porting an object to a new machine type will not be able to silently build an incorrect object that is missing a necessary mapfile definition.

        $if _sparc
        ...
        $elif _x86
        ...
        $else
        $error unknown machine type
        $endif

C language programmers will recognize that the syntax used for mapfile conditional input resembles that of the C preprocessor macro language. This similarity is intentional. However, mapfile conditional input directives are by design considerably less powerful than those provided by the C preprocessor. They provide only the most basic facilities required to support linking operations in a cross platform environment.

Among the significant differences between the two languages.

• The C preprocessor defines a full macro language, and the macros are applied to both the source text, and to the expressions evaluated by the #if and #elif preprocessor statements. Link-editor mapfiles do not implement a macro capability.

• The expressions evaluated by the C preprocessor involve numeric types, and a rich set of operators. Mapfile logical expressions involve boolean true and false values, and a limited set of operators.

• C preprocessor expressions involve arbitrary numeric values, possibly defined as macros, and defined() is used to evaluate whether a given macro is defined or not, yielding a true (nonzero) or false (zero) value. Mapfile logical expressions only manipulate boolean values, and names are used directly without a defined() operation. The specified names are considered to be true if they exist in the link-editor's table of known names, and false otherwise.

Those requiring more sophisticated macro processing should consider using an external macro processor, such as m4(1).

Directive Syntax

Mapfile directives exist to specify many aspects of the output object. These directives share a common syntax, using name value pairs for attributes, and {...} constructs to represent hierarchy and grouping.

The syntax of mapfile directives is based on the following generic forms.

The simplest form is a directive name without a value.

        directive;

The next form is a directive name with a value, or a white space separated list of values.

        directive = value...;

In addition to the “=” assignment operator shown, the “+=” and “-=” forms of assignment are allowed. The “=” operator sets the given directive to the given value, or value list. The “+=” operator is used to add the value on the right hand side to the current value, and the “-=” operator is used to remove values.

More complex directives manipulate items that take multiple attributes enclosed within {...} brackets to group the attributes together as a unit.

        directive [name] {
                attribute [directive = value];
                ...
        } [name];

There can be a name before the opening brace ({), which is used to name the result of the given statement. Similarly, one or more optional names can follow the closing brace (}), prior to the terminating semicolon (;). These names are used to express that the defined item has a relationship with other named items.

Note that the format for attributes within a grouping use the same syntax described above for simple directives with a value, with an assignment operator (=, +=, -=) followed by a value, or white space separated list of values, terminated with a semicolon (;).

A directive can have attributes that in turn have sub-attributes. In such cases, the sub-attributes are also grouped within nested {...} brackets to reflect this hierarchy.

        directive [name] {
                attribute {
                        subatribute [= value];
                        ...
                };
        } [name...];

The mapfile syntax grammar puts no limit on the depth to which such nesting is allowed. The depth of nesting depends solely on the requirements of the directive.